A minimum noise figure of a bipolar transistor is effectively reduced by reduction in the base-emitter junction capacitance CBE, an improvement in the direct-current gain β, and/or reduction in the sum, RB+RE, of a base resistance RB and an emitter resistance RE.
With respect to a SiGe-base bipolar transistor using SiGe as a base material, since the band gap of SiGe is narrower than the band gap of silicon (Si) or polysilicon (Poly-Si) serving as a material for an emitter, sufficiently large direct-current gain β can be achieved by increasing the impurity concentration of the base, and therefore, by reducing the base resistance RE. With respect to the SiGe-base bipolar transistor, the composition ratio of Ge in the base is increased in a direction from the emitter side toward a collector side, and thereby, the base-emitter junction capacitance CBE can be reduced. Consequently, a minimum noise figure of the SiGe-base bipolar transistor can be reduced compared with that of the conventional Si homo-junction bipolar transistor.
With respect to the SiGe-base bipolar transistor, the profile of the composition ratio of Ge has significant effect on the minimum noise figure. FIGS. 1 to 3 show examples of the profiles (the distributions of the composition ratio of Ge and the impurity concentration in the depth direction) in the vicinity of the bases of three SiGe-base bipolar transistors prepared based on a conventional technology. In these drawings, each of first solid lines 10, 20 and 30 indicates a concentration of arsenic (As), while arsenic is a dopant in an n-type emitter. Each of second solid lines 11, 21 and 31 indicates a concentration of boron (B), while boron is a dopant in a p-type base. Each of third solid lines 12, 22 and 32 indicates a concentration of phosphorus (P), while phosphorus is a dopant in an n-type collector. Bach of broken lines 13, 23 and 33 indicates a composition ratio (concentration) of germanium Ge. Each of first double-headed arrows 14, 24 and 34 indicates an emitter poly-Si region. Each of second double-headed arrows 15, 25 and 35 indicates a single crystal Si region. The single crystal Si region is of the p-type in an As-depo state (a state immediately after deposition), and finally becomes of the n-type because of diffusion of arsenic from the emitter poly-Si region 14, 24 or 34. Each of third double-headed arrows 16, 26 and 36 indicates a SiGe region (base region). Each of fourth double-headed arrows 17, 27 and 37 indicates a single crystal Si region (collector region) to serve an a collector.
In FIGS. 1 to 3, the emitter-base junction is a heterojunction. As shown in FIG. 1, the composition ratio of Ge in the base region 16 is 6% at the emitter-base junction. As shown in FIG. 2, the composition ratio of Ge in the base region 26 is 0% at the emitter-base junction. As shown in FIG. 3, the composition ratio of Ge in the SiGe region 36 is a constant 12%.
The direct-current gain β of the SiGe-base bipolar transistor has a dependence on the composition ratio of Ge at the emitter-base junction. That is, the direct-current gain β of the SiGe-base bipolar transistor having the profile shown in FIG. 3 is higher than that of the SiGe-base bipolar transistors having the profiles shown in FIGS. 1 and 2. The direct-current gain β of the SiGe-base bipolar transistor having the profile shown in FIG. 1 is higher than that of the SiGe-base bipolar transistor having the profile shown in FIG. 2.
On the other hand, with respect to the base-emitter junction capacitance CBE of the SiGe-base bipolar transistor, the capacitance of the SiGe-base bipolar transistor having the profile shown in FIG. 1 or 2 is smaller than that of the SiGe-base bipolar transistor having the profile shown in FIG. 3. In the profile shown in FIG. 1 or 2, the base has a gradient composition ratio of Ge, and in the profile shown in FIG. 3, the base has a constant composition ratio of Ge. The SiGe-base bipolar transistor having the profile shown in FIG. 1 exhibits the base-emitter junction capacitance CBE substantially equal to that of the SiGe-base bipolar transistor having the profile shown in FIG. 2.
An described above, with respect to the minimum noise figure, a smallest value is exhibited by the SiGe-base bipolar transistor having the profile shown in FIG. 1, wherein the compatibility is ensured between a higher direct-current gain β and a smaller base-emitter junction capacitance CBE.
A method for reducing the base-emitter junction capacitance CBE is known as a technology for further reducing the minimum noise figure of the SiGe-base bipolar transistor. In this method, a non-doped layer is provided between the base and the emitter.
FIG. 4 shows an impurity profile in the vicinity of the base of the SiGe-base bipolar transistor including a non-doped layer between the base and the emitter. In FIG. 4, a first solid line 40 indicates a concentration of arsenic (As), while arsenic is a dopant in an n-type emitter. A second solid line 41 indicates a concentration of boron (B), while boron is a dopant in a p-type base. A third solid line 42 indicates a concentration of phosphorus (P), while phosphorus in a dopant in an n-type collector. A broken line 43 indicates a composition ratio of Ge. A first double-headed arrow 44 indicates an emitter poly-Si region. A second double-headed arrow 45 indicates a single crystal Si region. The single crystal Si region is non-doped in an As-depo state (a state immediately after deposition), and finally a part thereof becomes of the n-type because of diffusion of arsenic from the emitter poly-Si region 44. A third double-headed arrow 46 indicates a SiGe region (base region). A fourth double-headed arrow 47 indicates a single crystal Si region (collector region) to so as a collector.
With respect to the bipolar transistor having the profile shown in FIG. 4, the base-emitter junction capacitance CBE is reduced because the composition ratio of Ge in the emitter-side portion of the base region 46 is decreased with increasing proximity to the emitter region side and the non-doped layer 45 is provided between the emitter region 44 and the base region 46. With respect to this bipolar transistor, the direct-current gain β is improved by increasing the composition ratio of Ge to 6% at the emitter region-side end of the base region 46. With respect to this bipolar transistor, reduction in the minimum noise figure is achieved by the reduction in the base-emitter junction capacitance CBE and the improvement in the direct-current gain β.
When the non-doped layer 45 is provided between the base region 46 and the emitter region 44, the direct-current gain β is reduced compared with that in the case where the non-doped layer 45 is not provided (in the case corresponding to FIG. 1). The reduction in the direct-current gain β brings about an increase in the minimum noise figure. However, when the thickness of the non-doped layer 45 is in the order of a few tens of nanometers or less, the influence of the reduction in the base-emitter junction capacitance CBE has the edge over the influence of the reduction in the direct-current gain β, and as a result, the minimum noise figure of the SiGe-base transistor including the non-doped layer 45 becomes smaller than the minimum noise figure of the SiGe-base transistor not including the non-doped layer 45.
In this manner, the SiGe-base bipolar transistor having the profile shown in FIG. 4 exhibits a minimum noise figure smaller than that of the SiGe-base bipolar transistor having the profile shown in FIG. 1.
However, a problem occurs in that manufacture is practically difficult with respect to a device having the profile shown in FIG. 4. This is because boron (indicated by the second solid line 41) diffuses into the emitter region 44 side due to heat history during manufacture of the device, while the boron is to be introduced into the base region 46, and a part of the non-doped layer 45 is converted to a part of the p-type base. That is, when a device is manufactured with the intention to have the profile shown in FIG. 4, the actually prepared device has the profile shown in FIG. 5.
In FIG. 5, a first solid line 50 corresponds to the first solid line 40 shown in FIG. 4, and indicates a concentration of arsenic (As), while arsenic is a dopant in an n-type emitter. A second solid line 51 corresponds to the second solid line 41 shown in FIG. 4, and indicates a concentration of boron (B), while boron is a dopant in a p-type base. A third solid line 52 corresponds to the third solid line 42 shown in FIG. 4, and indicates a concentration of phosphorus (P), while phosphorus in a dopant in an n-type collector. A broken line 53 corresponds to the broken line 43 shown in FIG. 4, and indicates a composition ratio of Ge. A first double-headed arrow 54 corresponds to the first double-headed arrow 44 shown in FIG. 4, and indicates an emitter poly-Si region. A second double-headed arrow 55 corresponds to the second double-headed arrow 45 shown in FIG. 4, and indicates a non-doped single crystal Si region in an As-depo state (a state immediately after deposition). A third double-headed arrow 56 corresponds to the third double-headed arrow 46 shown in FIG. 4, and indicates a SiGe region (base region). A fourth double-headed arrow 57 corresponds to the fourth double-headed arrow 47 shown in FIG. 4, and indicates a single crystal Si region to serve as a collector.
The single crystal Si region 55 is non-doped in an As-depo state (a state immediately after deposition), and partially, the base side and the emitter side are converted to of the p-type and n-type, respectively, due to heat history during manufacture of the device. The base side of the single crystal Si region 55 is partially converted to of the p-type, and thereby, the location of the bases-emitter junction moves in the side nearer to the emitter than the location of the SiGe—Si hetero-interface. As a result, with respect to the bipolar transistor having the profile shown in FIG. 5, the base-emitter junction capacitance CBE increased, the direct-current gain β is reduced, and the minimum noise figure is increased compared with that having the profile shown in FIG. 4. As described above, realization of the structure having the profile shown in FIG. 4 (ideal structure) is difficult, while the structure can ensure the compatibility between a mall base-emitter junction capacitance CBE and a high direct-current gain β.
The present invention was made in consideration of the above-described problems. It is a major abject of the present invention to provide a bipolar transistor having a small minimum noise figure by ensuring the compatibility between a small base-emitter junction capacitance CBE and a high direct-current gain β.